Diagnosis of hearing loss in children

Identifying hearing loss and deafness in an adult is easy enough. Most of the methods used by them are based on the respondent's answers to the sounds of certain tones and frequencies, as well as speech, delivered by the tuning fork or through the headphones. The curve derived from these subjective responses characterizes the state of the auditory function. However, these so-called psychophysical methods can be used in children not earlier than 4-5 years of life: at an earlier age the child, as a rule, is not able to give the correct answer. Meanwhile, it is at this and even earlier age that there is an urgent need to detect hearing loss, since it is closely related to the development of the child's speech function and intelligence.

It is known that 80% of hearing disorders occur in children in the 1-2 years of life. The main problem is that the belated diagnosis of hearing loss leads to untimely initiation of treatment, and consequently to late rehabilitation, delay in the formation of speech in the child. Modern concepts of the conductor's work and hearing aid are based on an earlier start of the training. The optimal age is 1-1.5 years, but if this time is lost, what happens in every third child, it is much more difficult to teach speech, and the child has more chances to become deaf and dumb. In this multifaceted problem, one of the most important questions is the early diagnosis of deafness, which is in the field of pediatrician and otorhinolaryngologist activities. Until recently, this problem remained an almost insoluble problem. The main difficulty lies in the need for an objective research based not on the answers of the child, but on other criteria that do not depend on his consciousness.

Method of unconditional reactions

The first group of such methods is simple, but, unfortunately, very inaccurate. The definition of hearing is based on the emergence of unconditioned reflexes in response to sound stimulation. According to the most diverse reactions (heart rate, heart rate, respiratory movements, motor and autonomic responses), indirectly they are judged whether the child hears the hearing or not. Some scientific studies show that even a fetus with about 20 weeks reacts to sounds, changing the rhythm of heartbeats. Very interesting are the data suggesting that the embryo can better hear the frequencies of the speech zone. On this basis, the conclusion is made about the possible reaction of the fetus to the mother's speech and the beginning of the development of the psychoemotional state of the child even during pregnancy.

The main contingent of application of the method of unconditioned reactions is newborns and infants. A hearing child should respond to sound immediately after birth in the first minutes of life. For the study used different sources of sound: sound, pre-calibrated sound level toys, trills, musical instruments, simple instruments - sound-proof meters, sometimes narrow-band and broadband noise. The intensity of sound is different, the general principle is that the older the child, the lesser the intensity of sound is needed to identify the reaction. So, at 3 months, the reaction is caused by an intensity of 75 dB, at 6 months - 60 dB, at 9 months, 40-45 dB is enough for the hearing child to appear. It is very important to correctly conduct and correctly interpret the results of the procedure: the study should be done 1-2 hours before feeding, as later reaction to sounds is reduced. The motor reaction may be false, that is, not at sounds, but simply at the approach of the doctor or the movement of his hands, so you should do some pauses each time. To exclude false positive reactions, a two-fold or three-fold identical response can be considered reliable. From many mistakes in determining the unconditioned response, the use of a specially designed hearing cradle for children is avoided.

The most common and studied types of unconditioned responses are cochleopalpelenal (blinking in response to sounds) and cochleopapillary reflex (pupil dilatation), motor orientation reflexes, disturbance of the rhythm of inhibition of the sucking reflex. Some responses can be objectively recorded, for example, changes in the lumen of the vessels (plethysmography), heart beat rhythm (ECG), etc. What are the positive aspects of this group of methods. They are simple, accessible in all conditions and therefore can be widely used in the medical practice of a neonatologist and pediatrician. However, their shortcomings should also be taken into account. First, a high intensity of sound and exact observance of research rules are necessary to exclude false-positive responses, mainly with unilateral hearing loss. Thus, we can find out only one question: whether the child hears (without the characteristic of the degree of hearing loss and its character). Although this is extremely important. With the help of this technique, you can try to determine the ability to localize the source of sound, which normally develops in children already from 3-4 months.

The group of methods of unconditioned reflexes can be widely used in practical work for screening diagnostics, especially in risk groups. If possible, all newborns and infants in the maternity hospital should conduct such studies and consultations, but they are considered mandatory only for the so-called risk groups for deafness and deafness. They include:

• causes that affect the auditory function of the fetus during pregnancy (congenital deafness and  deafness ); toxicosis, threat of miscarriage and premature birth, rhesus-conflict between mother and fetus, nephropathy, uterine tumors, mother's diseases during pregnancy, especially rubella, influenza, treatment with ototoxic drugs;
• pathological birth: premature, rapid, prolonged with the application of forceps, caesarean section, partial placental abruption, etc .;
• pathology of the early neonatal period: hyperbilirubinemia associated with hemolytic disease of the newborn, prematurity, congenital malformations, etc .;
• in the infancy and early childhood risk factors: sepsis, fever after delivery, viral infections (rubella, chicken pox, measles, mumps, influenza), meningoencephalitis, complications after vaccinations, inflammatory diseases of the ear, craniocerebral trauma, treatment with ototoxic drugs etc.

Maternal anamnesis

A major role for the initial judgment on the hearing condition of a child with suspected hereditary deafness is played by a maternal anamnesis. When interviewing a child's parents before the age of 4 months, they find out whether unexpected unexpected loud sounds awaken the sleeper, he shudders or cries: for the same age, Moro's reflex is characteristic . It is manifested by the dilution and the reduction of the hands (the reflex of grasping) and the stretching of the legs with strong sound stimulation.

For an approximate detection of hearing impairments, a congenital sucking reflex is used, occurring in a certain rhythm (similar to swallowing). Change of this rhythm with the sound effect is usually caught by the mother, this indicates the presence of hearing in the child. Of course, all of these orientation reflexes are better defined by parents. These reflexes are characterized by rapid extinction, which means that with frequent repetition the reflex can stop reproducing; from 4 to 7 months the child usually makes attempts to turn to the sound source, that is, already determines its localization, at 7 months he differentiates certain sounds, reacts, even if he does not see the source, attempts of speech responses (walking) begin at 12 months.

Risk factors play an extremely important role in the early diagnosis of hearing loss, and therefore, at the beginning of treatment or surdoeducation. It should be noted that deafness and deafness among newborns is noted on average at 0.3%, and in risk groups it increases almost 5-fold.

The method of conditioned reflex reactions

The second group of methods is based on the use of conditioned reflex reactions. For this, it is first necessary to develop an orienting reaction not only to the sound, but also to another stimulus, which supports the sound stimulus. So, if you combine feeding with a strong sound (for example, a bell), then in 10-12 days the sucking reflex will arise only in response to sound.

There are numerous methods based on this pattern, only the nature of reinforcement changes. Sometimes as it is used pain stimuli, for example, sound is combined with a prick or the direction of a strong air jet in the face. Such sound-reinforcing stimuli cause a defensive reaction (rather stable) and are used mainly to identify aggravation in adults, but can not be applied to children from humane considerations. In this regard, children use modifications of the conditioned reflex technique, based not on the defensive reaction, but on the contrary, on the positive emotions and natural interest of the child. Sometimes food is used as such reinforcement (sweets, nuts), however it is not harmless, especially with a large number of repetitions, when it is necessary to develop reflexes for different frequencies. That is why this option is more applicable for trained animals in the circus. The main method now used in the clinic is gaming audiometry, where the natural curiosity of the child is used as reinforcement. In these cases, sound stimulation is combined with displaying pictures, slides, video films, moving toys (for example, a railway), etc.

Scheme of the procedure: the child is placed in a soundproofed and isolated camera. On the examined ear, put on the earpiece, connected with any source of sound (audiometer). The doctor and the recording equipment are outside the cell. At the beginning of the study, high-intensity sounds are given in the ear, the child obviously should hear them, the child's hand is placed on the button, which is pressed by the mother or assistant when the beep sounds. After a few exercises, the child usually learns that the combination of sound with the click of a button results in either a change of pictures or a continuation of the demonstration of the video, in other words - the continuation of the game - and already presses the button independently when a sound appears.

Gradually, the intensity of the sounds is reduced. Thus, conditioned reflex reactions make it possible to reveal:

• one-sided hearing loss;
• determine thresholds of perception;
• give a frequency response of auditory function disorders.

Hearing by these methods requires a certain level of intelligence and understanding on the part of the child. Much depends on the ability to establish contact with parents, qualifications and skilful approach to the child by the doctor. However, all efforts are justified by the fact that from the age of three years in many cases it is possible to conduct a study of hearing and get a full-fledged characterization of the state of its auditory function.

Objective methods of investigating the auditory function

The objective methods of research of the auditory function include the measurement of acoustic impedance, that is, the resistance exerted by the sound-conducting device to the sound wave. Under normal conditions, it is minimal, at frequencies of 800-1000 Hz, virtually all sound energy reaches without the inner ear resistance, and the acoustic impedance is zero (tympanogram A). However, with the pathology associated with impaired mobility of the tympanic membrane, auditory ossicles, maze windows and other structures, some of the sound energy is reflected. It is also considered a criterion for changing the value of the acoustic impedance. In the external auditory canal, an impedance meter is tightly inserted, a sound of constant frequency and intensity, called probing, is fed into the closed cavity.

Three tests are used: tympanometry, static compilation and threshold of acoustic reflex. The first test gives an idea of the mobility of the eardrum and pressure in the cavities of the middle ear, the second - the ability to differentiate the stiffness of the auditory ossicles, the third, based on the contraction of the muscles of the middle ear, makes it possible to differentiate the damage of the sound-transmitting apparatus that is sound-conducting with the lesion. The data obtained with acoustic impedance measurements are recorded in the form of different curves on tympanograms.

Acoustic impedance measurement

There are some features that should be considered when conducting acoustic impedance measurements in childhood. In children of the first month of life, the study does not present great difficulties, since it can be carried out during a sufficiently deep sleep following the next feeding. The main feature at this age is associated with the frequent absence of an acoustic reflex. The tympanometric curves are recorded fairly clearly, although a large scatter of the amplitude of the tympanogram is observed, sometimes they are of a two-peak configuration. The acoustic reflex can be determined approximately from 1.5 to 3 months. However, it should be borne in mind that even in a state of deep sleep, the child experiences frequent swallowing movements, and the recording may be distorted by artifacts. That is why, for sufficient reliability, studies should be repeated. One should also take into account the possibility of errors in acoustic impedanceometry due to the compliance of the walls of the external auditory canal and changes in the dimensions of the auditory tube during crying or crying. Of course, you can apply anesthesia in these cases, but this leads to an increase in the thresholds of the acoustic reflex. We can assume that the tympanograms become reliable, starting from the age of 7 months, they give a reliable idea of the function of the auditory tube.

In general, acoustic impedance measurement is a valuable method of objective hearing research in infants and young children.

Some advantages also have the technique of recording the potential of the behind-the-ear muscle: using it, you can do without the use of sedatives and determine the hearing loss mainly at low frequencies up to 100Hz,

To the present revolution, research into the hearing in children has led to the development and introduction into clinical practice of the method of objective determination of auditory evoked potentials using computer audiometry. Already at the beginning of the XX century. With the discovery of electroencephalography, it was clear that in response to sound stimulation (stimulation) in different parts of the sound analyzer: cochlea, spiral ganglion, nuclei of the trunk and cerebral cortex - electrical responses (caused by auditory potentials) arise. However, they could not be registered because of the very small amplitude of the response wave, which was smaller than the amplitude of the constant electrical activity of the brain (beta, alpha, gamma waves).

Only with the introduction into the medical practice of computer technology it became possible to accumulate in the memory of the machine some insignificant answers to a series of sound stimuli, and then to summarize them (the total potential). This principle is also used in carrying out objective computer audiometry. Multiple sound stimuli in the form of clicks are fed into the ear, the machine memorizes and summarizes the answers (if, of course, the child hears), and then presents the overall result in the form of a certain curve. Objective computer audiometry makes it possible to conduct a hearing test at any age and even in a fetus from 20 weeks.

[1], [2], [3], [4], [5]

Electrochlearography

To obtain an idea of the location of the damage to the sound analyzer, which affects hearing loss (topical diagnosis), different methods are used. Electrochlearography is used to measure the electrical activity of the cochlea and spiral node. The electrode, by which electrical responses are removed, is installed in the area of the wall of the external auditory canal or on the tympanic membrane. This is a simple and safe procedure, however, the potentials that are removed are very weak, since the snail is far from the electrode. When necessary, the electrode is pierced by the tympanic membrane and placed directly on the promontorial wall of the tympanic cavity near the cochlea, that is, the site of potential generation. In this case, it is much easier to measure them, however, in children's practice, such a transtimpanal ECG is not widely spread. The presence of spontaneous perforation of the tympanic membrane greatly facilitates the situation. EKOG - the method is quite accurate and gives an idea of hearing thresholds, helps in differential diagnosis of conductive and non-irosensory hearing loss. Up to 7-8 years old it is under anesthesia, at an older age - under local anesthesia.

Thus, ECGG makes it possible to comprehend the state of the hair structure of the cochlea and the spiral node. Investigation of the state of the more deep-seated parts of the sound analyzer is performed using the definition of short-medium and long-latent auditory evoked potentials. The point is that the response to the sound stimulation of each department comes a little later, that is, it has its more or less prolonged latent period. Naturally, the reaction from the cortex of the cerebral hemispheres results in the latter and long latent potentials, namely, their characteristics. These potentials are reproduced in response to sound signals of sufficient duration and differ even in tone.

Latent period of short-latency - stem potentials extends from 1.5 to 50 mg / s, cortical from 50 to 300 mg / s. The source of the sound is sound clicks or short tonal parcels that do not have a tonal color, are fed through the headphones, bone vibrator. It is also possible to study by means of speakers in a free sound field. Active electrodes are placed on the mastoid process, attached to the lobe or fixed at any point of the skull. The study is performed in a soundproofed and shielded cell, in children up to 3 years old - in a state of medical sleep after the introduction of diazepam (Relanium) or 2% solution of chloral hydrate rectally in a dose corresponding to the body weight of the child. The study continues on average 30-60 minutes in the lying position.

As a result of the study, a curve containing up to 7 positive and negative peaks is recorded. It is believed that each of them reflects the state of a certain department of the sound analyzer: I - the auditory nerve, P-III - cochlear nuclei, trapezoid body, upper olive, IV-V - lateral loops and upper tubercles of the quadruple, VI-VII of the internal geniculate body.

Of course, there is a great variability in the responses of short-latent auditory evoked potentials, not only in hearing studies in adults, but also in each age group. The same applies to long-latent auditory evoked potentials - many factors need to be considered in order to compile an accurate picture of the state of the child's auditory function and the location of the lesion.

Electrophysiological methods for determining auditory function remain the most important and sometimes the only option for such a study of hearing in children of the period of the newborn, infancy and early childhood and are now becoming increasingly prevalent in medical institutions.

[6], [7], [8], [9], [10], [11], [12]

Acoustic emission

Literally recently, in the practice of hearing research in pediatrics begin to introduce a new method - the registration of delayed induced acoustic emission of the cochlea. These are extremely weak sound vibrations generated by the cochlea, they can be recorded in the external auditory canal with the aid of a highly sensitive and low-noise microphone. Essentially, this is the "echo" of the sound that is delivered to the ear. Acoustic emission reflects the functional capacity of the outer hair cells of the organ of Corti. The method is very simple and can be used for mass hearing tests, starting from the 3-4th day of the child's life, the study takes several minutes, and the sensitivity is high enough. 

[13], [14], [15], [16], [17], [18], [19], [20], [21], [22]

Hearing test by whispering and speaking

In older children, starting from 4-5 years, the same methods are used for hearing research as in adults. However, in this case, it is necessary to take into account certain features of childhood.

Thus, the study of hearing by whispering and conversational speech is very simple, and it is necessary to observe the exact rules for conducting it in order to obtain a correct judgment on the state of the hearing function of the child. Knowing this particular method for a pediatrician is especially important, since it can be performed by him alone, and identifying any hearing loss is already the basis for referral to a specialist. In addition, it is necessary to take into account some features of the psychological nature of children in the study of this technique.

First of all, it is very important that there is trust between the doctor and the child, since otherwise he will not answer the questions. It is better to give the dialogue the character of the game with the involvement of one of the parents in it. First, you can, to a child, to some extent interest him, for example, with such a question: "I wonder if you'll hear what I'm going to say in a very low voice." Usually, children are genuinely happy, if they can repeat the word, and are willingly involved in the research process. And, on the contrary, they become upset or withdrawn in themselves, if they do not hear the words from the first time. That's why children need to start research from a close distance, only then increasing it. The second ear is usually drowned out to exclude eavesdropping. In adults, everything is simple: apply a special ratchet. In children, its use usually causes fright, so muffling is caused by pressing on the tragus with its stroking, it is better for the parents to do it. The words proposed for repetition are not arbitrary, since in the norm, if high phonemes prevail, they are heard better and from a greater distance. From this point of view, it is better to use special tables containing grouped words on the basis of tonality and matched with the child's interests and intelligence.

Acuity of hearing is determined by the distance from which these words are perceived confidently (high tones up to 20 m whisper speech, low - from 6 m). The words are pronounced thanks to the reserve air (remaining in the lungs after the usual exhalation), to provide approximately the same intensity of sound, repeatedly, until complete recurrence.

The study of hearing with the help of whisper and colloquial speech using tables composed of words with predominantly low and high tones already gives the doctor some possibilities for differential diagnosis of the damage to the sound-transmitting and sound-receiving apparatus. Great opportunities are available to the pediatrician for the study of hearing with the help of tuning forks. The tuning fork was invented in the early 18th century. As musical instruments. They represent sources of pure low or high tone. The classical set of tuning forks makes it possible to investigate the hearing over the entire audible Tonskal from 16 to 20,000 Hz. However, for practical purposes it is quite enough to use two tuning forks: low-frequency and high-frequency. A low-frequency tuning fork examines the rumor through the air (air patency) and through the bone, placing it on the mastoid process (bone conduction). High-frequency tuning fork is used only to determine hearing through the air. This is due to the fact that in normal air conduction is twice as long as the bone, and high-frequency sounds with a small amplitude easily round the head of the child during research, falling into the other ear (listening with the second ear). That is why the study of hearing through the bone with a high-frequency tuning fork can give a false positive result. From the age of 4-5, the child understands well what they want from him, and usually gives reliable answers. The tuning fork is set in motion by the compression of its jaw or by their light impact, the duration of the sound is determined by the data of the tuning fork passport. In the study, both tuning forks are placed in the plane of the auricle, in order to avoid adaptation, it is removed from time to time and again brought closer to the ear. Decrease in the duration of perception of the tuning fork with low tones indicates the defeat of sound reproduction with high tones - eukovody perception. This is an important conclusion that a doctor can make. However, the use of the tuning fork (Cs) for its perception through air and bone greatly extends our possibilities in this respect. 

For a better understanding of the complex relationship between airborne and bone conduction, the following should be remembered: if a child does not hear a sound when airborne, this can be associated with two options. First: if there are diseases that disrupt the sound (sulfur plug, perforation of the tympanic membrane, rupture of the auditory ossicles, etc.). However, if the sound-conducting device is kept and conducts sound well, and only receptor cells (the second variant) are affected, the result will be the same: the child will not hear well, air conduction is shortened.

Thus, a reduction in air conduction may indicate a damage to the sound-conducting or sound-receiving apparatus.

The situation with bone conduction is different. There are almost no diseases accompanied by a decrease in bone marrow, so the shortening of bone conduction may be due only to the damage to the sound-receiving apparatus. Thus, the value of bone conduction is a characteristic of the state of the receptor function. Based on these concepts, it is easy to understand the experience of Rinne, in which air and bone conduction are compared. Normally, the child hears through the air about twice as much as through the bone, for example, through the air - 40 s, and through the bone - 20 s, this is denoted as a positive Rinne. The shortening of perception through the air (for example, for 30 s) while retaining its perception through the bone (or even some elongation) indicates the damage to the sound-conducting apparatus (Rinne becomes negative). Simultaneous shortening of bone and air conduction testifies to the disease of the sound-receiving apparatus (Rinne remains positive). Now the experience of Schwabach is also understandable, in which the bone conduction is compared between the child and the doctor (of course, if the latter has a normal hearing). "Shorter" Schwabach testifies to the defeat of the sound-receiving apparatus. These experiments are easily accessible for pediatricians and can give information about the condition of his hearing, which are fundamentally important for the future of the child.

Tonal threshold audiometry

Tonal threshold audiometry is the main method of hearing research in adults. In childhood, its use is possible from about 5 years of age. The meaning of audiometry is to determine the thresholds, that is, the minimum sound intensity that the patient perceives. These studies can be carried out over the entire audible frequency range (usually from 125 to 8000 Hz) and thus, as a result of the respondent's answers, get a full quantitative (in dB) and qualitative (in Hz) hearing loss characteristics for each ear individually. These data are recorded graphically in the form of curves (audiograms). Research is best done in a soundproofed chamber or a quiet room with the help of special instruments - audiometers. Depending on the purpose (practical, research), they are of varying degrees of complexity. It is quite enough for research tasks with the help of screening, polyclinic and clinical audiometers. With their help determine the bone and air conduction.

Of course, it's not bad when a child, placed in a soundproof camera (an unfortunate, but unfortunately, generally accepted term), behaves calmly. However, this is not always the case, and often accompanied by fright. That's why it's better to put it there with one of the parents or an assistant. The hearing room must have a home view, pictures, toys. Sometimes it is recommended to conduct a study of hearing simultaneously with several children, this calms them.

It is better to conduct audiometry in the morning, shortly after breakfast; The investigation begins, as a rule, with the definition of hearing on a hearing ear. However, in capricious children with a severe degree of hearing loss, one sometimes has to first examine the hearing ear that is worse. Adults determine the auditory function with small subthreshold intensities. It is better for children to give an intense tone at the beginning, and then gradually reduce it to the threshold, so they understand the research task better. 

The thresholds of air conduction are determined by the sound feeding through the headphones. In the study of bone conduction on the mastoid process, a special vibrator is installed. The exact definition of bone conduction is complicated by the fact that the sound reaches through the bones of the skull of both labyrinths, in addition, part of the sounds fall into the external auditory canal. If there is a big difference in hearing, listening may be better with the hearing ear, and the doctor receives false data. To exclude this, use a muffler that is better than the hearing ear, as if masked by a specially supplied intense noise. This should be done to exclude serious diagnostic errors that distort the overall picture of the child's hearing. The data obtained with tonal audiometry is recorded on the audiogram by conventional symbols: the right ear (o-o-o), the left ear (x-xx), the air conduction by a solid line, and the bone dashed line.

In addition to tonal audiometry, if necessary, in childhood, such studies as over-threshold, speech and ultrasound audiometry can be used.

Tone audiometry determines the faintest sound that the deaf ear begins to hear. If gradually and further amplify the sound, most patients will note the same gradual increase in perception. However, some at some level suddenly comes a sharp increase in volume. So, when talking to a deaf-eared person, he often repeats the phrases, but suddenly, with a slight increase in his voice, he says: "You do not need to shout like this, I can still hear everything." In other words, in these patients loudness increases rapidly, this phenomenon is also indicated: the phenomenon of an accelerated increase in loudness. This phenomenon occurs in patients with a local lesion of the cochlear hair device. He is given a lot of diagnostic value, especially it should be taken into account when selecting hearing aids. Modern audiometers are usually equipped for conducting above-threshold tests.

Speech audiometry

Speech audiometry is an advanced method of research with the help of whisper and colloquial speech. Its special advantage is the nature of the study. Because the perception of speech is one of the basic for the intellectual development of the child. That is why speech audiometry has found wide application as a prognostic technique for the work of the faculty, with hearing the best operations, the selection of hearing aids, reeducation, etc.

Through headphones or indoors speakers (free sound field) from the tape transfer individual words or phrase material. The child repeats the text transmitted to him in the microphone, and the doctor registers the answers. Usually, the following parameters are determined: the threshold of sound detection (in dB), the threshold of the initial intelligibility of speech (20% of words are normal with an intensity of 25 dB); 100% of the words are usually disassembled at 45 dB. As we have already mentioned, on the tape tape recorded speech tables, including some words or phrases, selected from sounds that are homogeneous in terms of acoustics.

To study hearing in hearing and deaf children these tables are not always applicable, because the vocabulary of such children is much poorer. For them, there is a specially selected dictionary and phrase material available for understanding by a deaf child.

Thus, speech audiometry has the following advantages over the usual study of whispered and colloquial speech: the text and diction of the researcher are constant, the volume of the delivered speech can be adjusted, the loss of hearing can be determined not in meters, but in decibels.

In some cases, after 6-7 years of age, ultrasonic audiometry may be used. Researches of domestic scientists showed: the ear perceives sound not only in the audible spectrum range up to 20 000 Hz, but also much higher, however only through the bone. Preservation of such a reserve of a cochlea that is not found on the usual audiogram testifies to some prospects for hearing aid, as well as hearing-improving operations (otosclerosis). For most children, the upper limit of audibility is not 200 kHz, but only 150 kHz.

Modern electrophysiological methods of hearing research, similar to ultrasound, are used not only in otorhinolaryngology, but largely neuropathologists, neurosurgeons and other specialists. They play an important role in the topical diagnosis of intracranial pathology: for tumors of the trunk and temporal lobe of the brain, stem encephalitis, temporal epilepsy, etc.